1. Field of the Art
This invention relates to a polymerization process for preparing, with high activity, high efficiency and high stability, propylene block copolymers having high rigidity and high impact resistance.
2. Related Art
Crystalline polypropylene has the advantageous properties of high rigidity and high resistance for heat, but has the shortcoming of low impact resistance which is particularly remarkable at low temperatures.
There has already been known a method for improving the above point, in which propylene, and ethylene or other olefin are polymerized by stages to give a block copolymer, as disclosed in Japanese Patent Publications No. 43-11230, No. 44-16668, No. 44-20621, No. 49-24593 and No. 49-30264, Japanese Laid-Open Patent Applications No. 48-25781, No. 50-115296, No. 53-35789 and No. 54-110072, and the like.
However, in the case where propylene and ethylene are polymerized in two stages or multiple stages, although the resulting polymer will have improved impact resistance, a large amount of low crystalline polymers are by-produced since the polymerization product contains copolymeric moieties.
In general, to improve the impact resistance of a block copolymer, the production rate of a rubber-like copolymer is increased. In this case, however, the amount of the by-product is increased, the polymer particles adhere to one another, and the polymer adheres to an inner wall of an apparatus which is used for the preparation of the polymer. As a result, it often becomes difficult to stably and continuously operate the apparatus over a long period of time.
Furthermore, the obtained polymer has the problems of a narrow molecular weight distribution, and of impaired formability or moldability upon forming or molding. In order to improve the above, there has been known a method in which a specific electron donor is added at the preceding polymerization stage or succeeding polymerization stage, as disclosed in Japanese Laid-Open Patent Applications No. 56-151713, No. 60-59139, No. 61-69821, No. 61-69822, No. 61-69823 and No. 63-43915. These techniques can contribute to, to some degree, a decrease in the production rate of by-product, and an improvement in the workability of the polymer produced. However, they have the problems of a decrease in the activity, and of an increase in the production cost because a large amount of additives have to be employed. On the other hand, the molecular weight of the rubber-like polymer becomes inevitably high when these techniques are applied. As a result, the workability of the polymer upon molding is improved, for instance, the formation of a flow mark is eliminated, but the coating property, adhesion property and the like may be impaired.
As other technique of adding an additive at the succeeding polymerization stage, there has been known a technique of employing alkyllithium or alkylmagnesium, as disclosed in Japanese Laid-Open Patent Applications No. 62-132912, No. 62-135509 and No. 2-117905. However, when an excessive amount of the additive is employed, it strongly acts as a catalytic poison, so that the polymerization activity of propylene is lost. For this reason, there is expected the development of a technique which can restrain the formation of a sticky or heptane-soluble product without affecting the molecular weight.